FIG. 1 depicts a typical steam turbine powertrain of a power plant PP 1 and the main associated steam lines. The steam turbine powertrain comprises high-pressure steam turbine 10, intermediate pressure steam turbine 11 and low-pressure steam turbine 12. High-pressure steam 13 (with or without supplementary firing) enters high-pressure steam turbine 10. Intermediate pressure steam 16 exits high-pressure steam turbine 10, flows through cold reheat line 14 to reheat device 17 (which may be part of the heat recovery steam generator), is reheated and flows then through hot reheat line 15 to intermediate pressure steam turbine 11. Finally, low-pressure steam 18 exits intermediate pressure steam turbine 11 and enters low-pressure steam turbine 12. If present, the degree of supplementary firing (SF) employed is limited by the inlet pressure of high-pressure steam turbine 10 and the associated water/steam cycle pressures.
Document U.S. Pat. No. 6,442,924 teaches, within the context of supplementary firing, the art of injecting the steam generated by the duct burners, into locations downstream of the inlet of high-pressure steam turbine 10 (see power plant PP 2 in FIG. 2). This measure permits a high-pressure steam turbine of a given design to always run at its design point (i.e. approximately constant pressure and mass flow rate), thereby performing optimally both with and without supplementary firing. FIG. 2 illustrates the various points A, B and C at which all of the additional steam raised by a supplementary firing can be injected (see arrows). However, this document does not describe increasing the mass flow rate through the high-pressure steam turbine 10.
FIG. 3 shows a typical arrangement of the so-called ISCC (Integrated Solar Combined Cycle) power plant PP3, wherein the solar field of a solar thermal plant 20 (preferably via a central receiver technology) is used to generate additional solar steam 21, which is then injected into (or slightly upstream of) the high-pressure steam turbine 10. An example of an ISCC is disclosed in document EP 2 372 116 A1.
Configuring a combined cycle plant to operate in tandem with a solar steam generator and maximizing the overall plant flexibility leads to the identification of the issues 1-3 listed below, whose resolution (a) increases the performance of the plant and (b) ensures high degrees of flexibility:                1. The solar fields might produce more steam than can be accommodated by the high-pressure steam turbine 10. For instance, at 100% gas turbine load, a limited amount of solar steam 21 can be typically added so as to remain within the pressure limit at the inlet of the high-pressure steam turbine 10. Since oversizing the high-pressure steam turbine 10 leads to performance reductions in the absence of solar steam 21, a significant quantity of solar steam 21 is rejected. This is a sub-optimal use of the expensive solar field.        2. It is potentially possible to run the ISCC in pure solar mode; i.e. the gas turbines (not shown in the Figures) are switched off and the steam turbines 10, 11 and 12 are driven by steam that is generated exclusively by the solar field. Unfortunately, the cold reheat steam temperature falls during the long reheater section, resulting in steam that is saturated, or only a few degrees above saturation, at the inlet of the low-pressure steam turbine 12. The intolerance of such wet steam by the intermediate pressure steam turbine 11 (exit) and the low-pressure steam turbine 12 (inlet), therefore, precludes pure solar operation.        3. The solar steam quality and quantity cannot be constantly matched to the requirements of the steam turbine. This is problematic during planned transients (e.g. changing of plant load, changing of solar contribution) as well as during unplanned perturbations (e.g. rapid load changes, variable cloud cover), because large changes (>50° C.) at the inlet of any of the steam turbine modules resulting in tripping of the steam turbine powertrain.        